The International Atomic Energy Agency said yesterday it would investigate US claims that Syria had tried to build a nuclear reactor with North Korean help, but "deplored" the delay in handing over the evidence to its inspectors.

The IAEA director general, Mohamed ElBaradei, was also sharply critical of Israel for bombing the site of the alleged reactor last September, near al-Kibar on the Euphrates in eastern Syria, saying the air strike undermined "the due process of verification that is at the heart of the non-proliferation regime".

The Bush administration sent the IAEA pictures of the alleged reactor at the same time as presenting them to Congress on Thursday. Washington claimed the facility was weeks from being operational.

A commentary accompanying a slideshow of photographs and computer graphics said the reactor "would have been capable of producing plutonium for weapons, was not configured to produce electricity, and was ill-suited for research".

The presentation also provided evidence of North Korean involvement in the reactor's construction and said that "in the past 35 years" only North Korea had built gas-cooled reactors of the "Magnox" type that the Kibar construction resembles.

Syria's president, Bashar al-Assad, denied the site was suspect. "Is it logical for a nuclear site to be left without protection and not guarded by anti-aircraft guns?" he told the Qatari paper al-Watan. "A nuclear site under the watch of satellites in the middle of Syria in the desert and in an open location?"

The IAEA had asked to visit the site of the September air strike last year but was turned down by Damascus. IAEA officials said a new request would be submitted.

The IAEA said: "The agency will treat this information with the seriousness it deserves and will investigate the veracity of the information." It added that under its 1992 agreement with the IAEA, Syria is obliged to report "the planning and construction of any nuclear facility".

However, ElBaradei's real outrage is reserved for the US. "The director general deplores the fact that this information was not provided to the agency in a timely manner, in accordance with the agency's responsibilities, to enable it to verify its veracity and establish the facts," the statement said.

Independent analysts at the Institute for Science and International Security (Isis) in Washington described the evidence as "dramatic confirmation" that the Kibar site was a nuclear reactor.

However, Isis said it was unclear where Syria would have acquired the uranium fuel necessary to power the reactor.

"The lack of any identified source of this fuel raises questions about when the reactor could have operated, despite evidence that it was nearing completion at the time of the attack," an Isis report said.

It also argued the evidence should not be used as a justification for military action against Syria or to derail "six-party talks" over North Korea's nuclear programme.